I play the games on PS3, but I've heard they feel really, really good to play on the Vita (I plan to get them on the Vita eventually as well, because who wouldn't love Miku on the go?!). Though I think the important question is are there other games you want for the Vita? I'm not sure if getting a Vita for just two games would really be worth it. I guess if you don't have a PS3 and you really want to be able to play the Project DIVA games it could be, but you can also get a PS3 for about the same price or possibly even cheaper than a Vita.
Though if playing on the go is what you're aiming for, then like I said, look to see what other games the Vita has that you might be interested in before making a decision.
As for the Vita itself, I love it. I think it's an amazing system and I prefer it over the 3DS (though I still love the 3DS too). It has a lot of games that fall right into my area of interest (JRPGs). If you're also a fan of JRPGs, then the Vita could very well be worth getting for you.